fix undefined behaviour in seeder (select() aliasing)

Summary:
I don't see the restrict-ness documented in man pages, but the previous
usage of select() here was triggering an aliasing warning.

../../src/seeder/bitcoin.cpp:294:36: warning: passing argument 2 to restrict-qualified parameter aliases with argument 4 [-Wrestrict]
         int ret = select(sock + 1, &set, nullptr, &set, &wa);
                                    ^~~~           ~~~~

Test Plan: make check

Reviewers: Fabien, jasonbcox, #bitcoin_abc, deadalnix, nakihito

Reviewed By: #bitcoin_abc, deadalnix, nakihito

Differential Revision: https://reviews.bitcoinabc.org/D4618